Wafer storing containers have been traditionally used that store a plurality of wafers in slots provided therein and permit the stored wafers to be unloaded from or reloaded into the container through a forward opening.
Such a wafer storing container is transported by a robot and placed on an opening/closing device (hereinafter referred to as the load port) adapted to open or close the lid of the wafer storing container. This load port is an additional device attached, for example, to a wafer treatment apparatus adapted to perform a predetermined treatment of the wafers.
Incidentally, there are three anomalous states in which the wafers stored in the wafer storing container may be. The first anomalous state is a missing wafer in a slot. Further, the second anomalous state is a plurality of wafers stacked one on top of the other in a slot. Still further, the third anomalous state is a wafer stored in a slanting position in uneven slots, i.e., a so-called cross-slotted wafer.
A robot attempting to take out a wafer in one of the above three states is unable to do so unless the robot knows the state in which the wafer is. For this reason, robots having a wafer detecting apparatus adapted to detect the storage state of wafers have been proposed in related arts (refer, for example, Patent Documents 1 and 2).
The wafer detecting apparatus disclosed in Patent Document 1 is a light-transmitting apparatus having a transmitter on the right or left side of wafers to shine light and a receiver on the other side to receive light shined from the transmitter. Therefore, if light from the transmitter which would otherwise reach the transmitter is blocked by the wafers, it is possible, by detecting the vertical width of blocked light, to verify that wafers are stacked one on top of the other and that a wafer is cross-slotted. Further, if there is an area where light is not blocked at all, it is possible to verify that there is a missing wafer in a slot.
Incidentally, a wafer may bend or warp by its own weight when thermally treated during semiconductor manufacturing. If such a deformed wafer is detected by the light-transmitting wafer detecting apparatus disclosed in Patent Document 1, the wafer may be erroneously detected as a plurality of wafers stacked one on top of the other as described above because of a larger vertical width of blocked light despite the fact that there is only one wafer. It should be noted that the larger the size of a wafer (e.g., 300 mm to 450 mm), the further more often the above erroneous detection will Occur.
For this reason, a light-reflecting detecting apparatus disclosed in Patent Document 2 has been proposed that is designed to detect reflected light from a wafer that has struck the edge surface of the wafer rather than detecting transmitted light as described above so as to ensure high accuracy in the detection of the deformed wafer. This detecting apparatus has an illumination light source at the upper area of a detecting head and an imaging device at the lower area thereof. Therefore, light from the illumination light source is shined on the edge surface of a wafer, and reflected light from the edge surface is imaged by the imaging device. This makes it possible to positively detect not only a missing wafer and a plurality of wafers stacked one on top of the other but also a cross-slotted wafer described above even in the event of bending or warping of a wafer described above. However, this detecting apparatus has three drawbacks described below.